It is now important to be able to select and purify cells from the in vitro marrow-thymus culture system with specific memory properties. Density gradient separation and nylon wool columns will be used in this work. Attempts will be made to further characterize surface molecular changes on differentiating T lymphocytes and to ascribe functional explanations to these membrane modifications. Thymic factors that influence the various steps of differentiation will be defined as much as possible. Hapten modified tumor cells will be used as antigens in our memory studies and effort will be made to characterize the functional lymphoid populations and the regulatory systems that influence their maturation.